Solid state semiconductor light source has a smaller size, and higher efficiency and greater adaptability, thus has become a very important energy saving product in applications of optical communication, industrial sensors and white-emitting luminescence. The conventional light-emitting diode (LED) generates spontaneous emission through recombination of electrons and holes. The light being generated is a random light with diverse phases, polarities and radiating directions, hence also called incoherent light (referring to FIG. 1A). On the other hand, semiconductor laser light is generated through repetitive optical amplification according to the principle of laser action that has spontaneous emission taking place in a cavity resonator between two mirrors at two ends to generate standing wave oscillation and produce stimulated emission. The light being generated is coherent light with the same phase, polarity and radiation direction (referring to FIG. 1B). FIG. 2 illustrates a Fabry-Perot laser made of LEDs in a heterogeneous structure. It has an anode 200, a p-type AlGaAs semiconductor 201, a n-type AlGaAs semiconductor 202, a confinement layer (also called cladding layer) 206 which is an active layer made of a p-type GaAs semiconductor interposed between the semiconductors 201 and 202. Carriers are injected and confined in the active action layer to facilitate light generation. It also has an n+-type GaAs substrate 203 and a p+-type GaAs 204 and a cathode 205. Because of the repetitive optical amplification of the laser action that generates light in the same direction, light extraction efficiency (ηex) is higher. The commonly used wall-plug efficiency (ηwp) of the LED is the ratio of optical output power and input power of the LED. Equation ηwp=ηint×ηex×ηv is applicable here, where ηv is voltage efficiency. ηv=hυ/qV. ηint is the internal quantum efficiency which is the ratio of the number of photons and the number of coupled electrons and holes. Another equation ηint=(ledPopt/hυ)/(I/q) can be applied, where h is Plank's constant, υ is photon frequency, q is electric charge, V is voltage, I is current, and ledPopt is optical output power of the LED.
The following equation also can be derived:
                                                                        η                wp                            =                            ⁢                                                η                  int                                ×                                  η                  ex                                ×                                  η                  v                                                                                                        =                            ⁢                                                                    (                                                                                            P                          opt                                                                                led                                                                            /                        h                                            ⁢                                                                                          ⁢                      υ                                        )                                    /                                      (                                          I                      /                      q                                        )                                                  ×                                  η                  ex                                ×                                  (                                      h                    ⁢                                                                                  ⁢                                          υ                      /                      qV                                                        )                                                                                                        =                            ⁢                                                (                                                            P                      opt                                                                    led                                                                ×                                          η                      ex                                                        )                                /                IV                                                                        (        1        )            
Hence for a given input power IV, to get a higher wall-plug efficiency ηwp, the internal quantum efficiency has to increase to achieve a higher optical output power ledPopt and higher light extraction efficiency ηex.
The relationship of lighting power generated by the conventional solid state semiconductor and current (namely P-I characteristics) is shown in FIG. 3 which illustrates characteristic comparisons of an LED in a spontaneous emission zone and a laser diode in stimulated emission. For an LED in a special structure such as DFB laser or Fabry-Perot laser, when the forward-biased current reaches the starting current, additional injected current is totally transformed to laser light emitting from the semiconductor. Assumed that a unit current 1iu is injected into the spontaneous emission zone before the laser action taking place, and optical output power is 1pu for comparison, injecting two times of current 2iu in the laser action zone can generate 16pu of optical output power, thus an extra power about 15pu can be generated. Based on the equation (2) set forth above, the following equation also can be derived:I−Ith=eUB(Ne−No)Np=(eU/τp)Np  (2)
where I is the injected current after started, Ith is the starting current, U is the volume of the active action layer, B is the vanishing probability of the injected electrons resulting in the stimulated emission, e is the charge amount of the electrons, Ne is carrier (electrons) density in the active action layer of the laser diode after DC power is injected, No is the minimum starting carrier (electrons) density, Np is the photon density of the stimulated emission, and τp is the average lifetime of the photons that can be indicated according to the equation below:τp=(n/c)(α+L−1 ln R−1)  (3)
where n is the refractive index of the active action layer, c is light speed, α is the value of light absorption coefficient per unit length in material of the active layer, L is the length of the resonate cavity, and R is the reflectivity of the two end surfaces.
Referring to equation (2), when the current is greater than the starting current Ith, additional injected current is totally transformed to stimulated emission, The lighting power is shown in FIG. 3 by the slope of the stimulated emission zone, and is proportional to (I−Ith). The equation (2) can be differentiated to derive another equation as follow:dNp/dI=τp/eU  (4)
It shows the slope of the photon density of the stimulated emission generated by injection of current in the laser diode. It indicates that the longer the average life span τp of the stimulated emission photon, the greater the slope becomes. The equation (3) also shows that the smaller the absorption α of the active action layer, and the longer the resonate cavity, the greater the slope becomes. Namely, a greater optical output can be generated.
Light emission efficiency of the laser diode can be indicated in four approaches: internal efficiency (ηi), differential quantum efficiency (ηd), total device efficiency (ηt) and laser efficiency (ηl). The laser efficiency ηl is defined the same as the LED power efficiency ηwp, but with different unit indications, and are different from light emission efficiency of the LED.
The internal efficiency is the percentage of the number of photons Np in stimulated emission generated by a forward-biased voltage against the number of injected electrons Ne, namely:ηi=(Np/Ne)×100%  (5)
The differential quantum efficiency is the percentage of the number of photons in stimulated emission against the number of injected electrons in a unit time, namely:
                                                                        η                d                            =                            ⁢                                                (                                      ⅆ                                          (                                                                                                    P                            opt                                                                                      ld                                                                                  /                          h                                                ⁢                                                                                                  ⁢                        υ                                            )                                                        )                                /                                  ⅆ                                      (                                          I                      /                      ⅇ                                        )                                                                                                                          =                            ⁢                                                (                                                                                    ⅆ                        ld                                            ⁢                                              P                        opt                                                              /                                          ⅆ                      I                                                        )                                /                Eg                                                                        (        6        )            
where Eg is the minimum energy gap (Eg=Ec−Ev) of the emission wave length of a selected light-emitting material, ldPopt is the lighting power of the laser diode as shown in FIG. 3, the slope is:tan α=dldPopt/dI=ηd×Eg  (7)
The total device efficiency is defined as the ratio of the number of emitting photons against the number of injected electrons, and can be indicated by equation (8) below:
                                                                        η                t                            =                            ⁢                                                (                                                                                    P                        opt                                                                          ld                                                                      /                      h                                        ⁢                                                                                  ⁢                    υ                                    )                                /                                  (                                      I                    /                    ⅇ                                    )                                                                                                        =                            ⁢                                                                    P                    opt                                                              ld                                                          /                  I                                ⁢                                                                  ⁢                Eg                                                                                        =                            ⁢                                                η                  d                                ⁡                                  (                                      1                    -                                          (                                                                        I                          th                                                /                        I                                            )                                                        )                                                                                        (        8        )            
The laser efficiency is defined as the ratio of light emission power against the input electric power, namely:ηl=ldPopt/IV=ηt×(Eg/V)  (9)
where V is the voltage applied to the laser diode. The laser efficiency is indicated the same as the power efficiency of the LED. Although the wall-plug efficiency of the LED ηwp (ηwp=(ledPopt×ηex)/IV) and the laser efficiency ηl (ηl=ldPopt/IV) of the laser diode are derived based on the ratio of the optical output power and the input power, the slope of the laser diode is much greater than the LED. The main difference is that: the extraction efficiency of the spontaneous emission ηex is lower. As a comparison, when the current in the resonate cavity is greater than the starting current Ith, the additional current (I−Ith) (namely the extra carriers being injected) resulting from the stimulated emission is totally transformed to the stimulated emission. Hence the slope increases significantly. Moreover, according to equation (2), increasing U also increases the volume of the active action layer, and more electrons are injected to boost output power. As a result, maximum optical energy can be obtained or energy saving effect can be achieved. Thus getting a higher extraction efficiency through standing wave oscillation of the resonate cavity and increasing the volume of the active action layer are a preferable choice of the light emitting structure. However, transforming the laser light of maximum power efficiency to high photoelectric energy for illuminating purpose still has drawbacks, notably:
1. The lighting spot is concentrated and cannot illuminate a greater area. And concentration of energy also damages the projecting object, such as hurting retina or creating the risk of uncontrollable burning.
2. The conventional laser chips are made by forming epitaxy on a semiconductor wafer, then performing cutting. The laser chips contain an active action layer capable of emitting light, but being formed at a limited volume. Hence using the initial optical energy of the limited spontaneous emission can trigger only limited amplification in the photoelectric transformation of the laser amplification already done. Moreover, the cutting operation to form the laser crystals has to be done in the expensive integrated circuit manufacturing process and manufacturing processes of plane deposition or epitaxy forming on the expensive wafer. As a result, the production cost is higher. All this makes mass production to expand applications difficulty.
If the aforesaid drawbacks can be overcome, the high efficiency solid state laser can be adopted for lighting use. Moreover, the problems of the low external quantum efficiency of the spontaneous emission of the solid state lighting equipment and the loss caused by internal thermal absorption occurred to the conventional white-emitting LED also can be resolved. Then using the semiconductor for white-emitting luminescence can be truly realized.